Conventionally, an example of a system for performing an automatic volume control depending on a level of noise in a vehicle has been disclosed by Japanese Patent No. 3352946. This system has an adaptive sound quality/volume control apparatus that provides an effect of controlling a sound quality and a volume in a manner natural to human ears by automatically controlling the volume, during the switching between sound sources as an input source, via a switch for changing a correction value for volume control.
For example, a system shown in FIG. 6 will be exemplarily described. In FIG. 6, switch 12 is designed to allow, while a sound source signal is being outputted from sound source A 10 in FIG. 6, sound from sound source A 10 to be inputted to the system. In this status, a microphone signal collected by microphone 16 and a sound source signal from sound source A 10 are inputted to control amount calculator 17. Then, sound control section 13 is controlled based on a control amount previously determined for sound source A 10 based on an assumed noise level. When sound source B 11 is selected in this status, switching controller 18 controls control amount calculator 17 and switch 12. Based on a control amount previously determined for sound source B 11, control amount calculator 17 controls sound control section 13 and switch 12 allows an input from sound source B 11 to enter the system. By the control as described above, control amounts to sound source A 10 and sound source B 11 are always calculated and a switching between the control amounts is performed when a switching between the sound source signals is performed. Thus, a sound quality and a volume can be adjusted by an optimal control amount even when the signal level of the sound source suddenly increases or decreases due to the switching between sound source signals, thus providing sound control in a manner natural to human ears. As described above, the conventional adaptive sound quality volume control apparatus has provided an automatic sound control in response to the switching between sound sources as an input source.
However, with the recent advent of multichannel and multimedia such as multichannel audio (e.g., Digital Versatile Disc (DVD)), different sound source signals may be inputted to the system from even a single sound source. This has caused, when a sound source is a DVD and a DVD linear pulse code modulation (PCM) signal or a DVD 5.1 ch (channel) signal is reproduced, a situation where the automatic sound control suppresses, if the control method is not changed, a dialog sound outputted from a center channel by the reproduction of a DVD 5.1 ch signal for example from being caught by human ears despite the intention or causes an unnatural control to human ears.